chore(frontend): remove the now-dead .error CSS class

Every inline error paragraph that used it has been migrated to the
banner system across the preceding tasks.

frontend/src/App.css

@@ -269,10 +269,6 @@ button:disabled {
   color: #9aa0ab;
 }
 
-.error {
-  color: #f87171;
-}
-
 .review-list {
   list-style: none;
   padding: 0;